[mips][fastisel] Consider soft-float an unsupported floating point mode

Treat soft-float as unsupported for fast-isel. Additionally, ensure we check
that lowering f32 arguments also considers the case of soft-float mode.

Reviewers: ehostunreach, vkalintiris, zoran.jovanovic

Differential Review: https://reviews.llvm.org/D24505

llvm-svn: 283209